This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 0004695-3 filed in Sweden on Dec. 19, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to coated cemented carbide cutting tool insert. The cemented carbide insert is based on WC, cubic carbides and has a Co-binder phase enriched surface zone. By alloying the cemented carbide with Mo, the performance has been improved particularly when used for turning at conditions causing intermittent thermal and mechanical load in stainless steel.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
High performance cutting tools have nowadays to possess high wear resistance, high toughness properties and good resistance to plastic deformation. Improved resistance to plastic deformation of a cutting insert can be obtained by decreasing the WC grain size or by lowering the overall binder phase content, but such changes simultaneously result in significant loss in toughness properties.
Methods to improve the toughness behavior without loss in plastic deformation by so called gradient-sintering techniques are known. The gradient consists of thick essentially cubic carbide free and binder phase enriched surface zones ( less than 50 xcexcm) of the cemented carbide inserts e.g., through U.S. Pat. Nos. 4,277,283, 4,610,931, 4,830,930, 5,106,674 and 5,649,279. Such inserts with essentially cubic carbide free and binder phase enriched surface zones are extensively used today for machining of steel and stainless steel. These patents are examples of the importance of the substrate composition within the surface zone for cutting performance. The properties of the insert such as resistance to plastic deformation and toughness behavior have to be balanced for optimal performance during machining to ensure long and stable tool life.
There are also ways to balance the plastic deformation resistance and toughness properties to a certain extent by controlling the composition of the surface zone by employing special sintering techniques or alloying elements, e.g., U.S. Pat. Nos. 5,484,468, 5,549,980, 5,729,823 EP-A-560 212 or EP-A-569 696. The characteristics of all the above-mentioned patents are that the surface zones are essentially cubic carbide free and binder phase enriched, i.e., they consist of WC and Co. Such surfaces zones give the insert good edge toughness but makes the insert less sufficient when working conditions are causing thermal and mechanical load to the insert.
It is therefore an object of the present invention to provide a cemented carbide insert with improved properties for turning when the temperature and mechanical load is varying without losing resistance to plastic deformation and edge toughness.
A coated cutting tool insert has a cemented carbide substrate having a surface zone and an inner portion and a coating. The substrate comprises WC, 5-15 wt. % Co, 0.5-4 wt. % Mo, 1-10 wt. % cubic carbides, and a binder phase enriched surface zone essentially free of gamma phase and the surface zone has a Mo content 0.9-1.1 of that in the inner portion.
In one aspect, the WC has grains having a duplex structure made up of a core and a surrounding rim containing Mo.
In an additional aspect, the cobalt binder phase has a CW-ratio of 0.72-0.94, the CW-ratio expressed as CW-ratio=Ms/(wt. % Coxc3x970.0161) where Ms is the measured saturation magnetization of the cemented carbide and the wt. % Co is the weight percentage of Co in the cemented carbide.
In a further aspect, the coating comprises  less than 1 xcexcm TiN, 1-5 xcexcm MTCVD-TiCN, 1-3 xcexcm xcexa-alumina, and  less than 1 xcexcm TiN.
In a still further aspect, the cubic carbides are selected from the group consisting of TiC, TaC and NbC.